1. Field of the Invention
The invention relates to pistons for internal combustion engines and to methods for their manufacture.
2. Review of the Prior Art
In the combustion chamber of an internal combustion engine whether located away from the piston or bounded by the piston, heat is generated on ignition of the fuel and the purpose of the engine is to convert this heat efficiently into usable engine power. Since the components surround the combustion chamber (the piston, the cylinder or cylinder liner, the piston head, and inlet and outlet valves) are made from heat conductive materials, some of the heat created on ignition will be lost by heat conduction through these parts. Any reduction of heat lost in this way may be used to improve the efficiency of the engine as well as increasing the temperature in the combustion chamber.
The components surrounding the combustion chamber must, however, be capable of resisting the temperatures achieved in the combustion chamber without long term damage being caused. Since pistons are customarily made from aluminium or aluminium alloy this can cause problems, if the combustion chamber temperature is raised, since these materials are not readily able to withstand high temperatures. In addition, since the heat is conducted axially down the piston, it is necessary to ensure that all the various parts of the piston, such as the piston rings, the piston ring grooves and the skirt are all suitably heat resistant and this can result in the need to use complex alloys which are expensive and, in the case of piston ring grooves, to reinforce them. In addition, where a number of piston rings are provided, the top piston ring closest the combustion chamber must be made of a material able to resist high temperatures and must be made with natural radial springiness to urge it against the cylinder or liner wall since the temperatures encountered do not permit the use of a separate ring expander.